Preparation And Electrochemical Properties Of Sodium Titanium Phosphate As An Anode Material For Aqueous Sodium Ion Batteries

Aqueous sodium-ion batteries have many advantages,such as high ionic conductivity,safety,non-spontaneous combustion,low cost of raw materials and low requirement of assembly conditions.This kind of battery system can meet the needs of large-scale energy storage,which attracts more and more attention of researchers.NaTi₂（PO₄）₃ is a kind of material with sodium superionic conductor structure（NASICON）.Its three-dimensional open frame structure is conducive to the diffusion of Na⁺.The charging and discharging platform is about-0.82 V（vs.Ag/AgCl）.It has a theoretical specific capacity of 132.8 mAh g^-1 and can be used as the negative electrode material for aqueous sodium ion batteries.However,its poor electronic conductivity makes its rate performance worse.In order to improve the electrochemical properties of the materials,NaTi₂（PO₄）₃ was modified by modification of high conductivity materials and nitriding treatment of the surface of the materials,and different systems of aqueous sodium ion batteries were assembled.The main research contents are as follows:（1）The pure phase NaTi₂（PO₄）₃ material was synthesized by solvothermal method and calcined in ammonia atmosphere.TiN-coated NaTi₂（PO₄）₃ material was successfully synthesized.Nitriding process does not change the crystal structure of NaTi₂（PO₄）₃,but part of Ti⁴⁺can be reduced to Ti³⁺,forming an amorphous TiN layer on the surface of the material.After nitriding treatment,the electronic conductivity of the material is significantly improved.By adjusting the nitriding time,we synthesized NaTi₂（PO₄）₃ material with the best nitriding effect,and clarified the effect of nitriding process on NaTi₂（PO₄）₃ material.Compared with pure NaTi₂（PO₄）₃ material,TiN-modified NaTi₂（PO₄）₃ material has better rate performance.The specific capacity of TiN-modified NaTi₂（PO₄）₃material can reach 131.9 mAh g^-1 at 2 C current density,and it remains 92 mAh g^-1 after 100 cycles.（2）Pure phase NaTi₂（PO₄）₃ material was synthesized by solvothermal method,and then a layer of polydopamine was polymerized on its surface.Carbon-coated NaTi₂（PO₄）₃ material was synthesized by calcination at high temperature,which solved the problem of low electronic conductivity of pure phase material.We also synthesized the cathode material Na₂NiFe（CN）₆ by liquid phase precipitation and assembled NaTi₂（PO₄）₃-Na₂NiFe（CN）₆ aqueous sodium ion battery.The working voltage range of the battery is 0.2¹.6 V,the specific capacity of the first cycle is 76 mAh g^-1.After 1000 cycles,there is still a specific capacity of 74 mAh g^-1,which shows excellent cycle stability.（3）NaTi₂（PO₄）₃ anode material and MnO₂ cathode material were synthesized.The electrochemical properties of the two materials in Na₂SO₄,MgSO₄ and Na₂SO₄-MgSO₄ mixed aqueous solution were studied.The results show that MnO₂ has excellent electrochemical performance in electrolyte containing Mg²⁺and its specific capacity can reach about 100 mAh g^-1.Na⁺and Mg²⁺can be intercalated/deintercalated reversibly from the cathode MnO₂ material at the same time,while the NaTi₂（PO₄）₃ anode material can only intercalate/deintercalate Na+.Based on this,we constructed an aqueous Sodium-Magnesium hybrid ion battery system.The anode electrode is carbon-coated NaTi₂（PO₄）₃ material,and the cathode electrode is MnO₂/CNTs composite material.The electrolyte used is Na₂SO₄-MgSO₄ mixed aqueous solution.The working voltage of the battery is about 1.4 V and its capacity can reach 97 mAh g^-1.